Re: [Boost-users] [mpl] Implementing eraseVal

Scott Meyers ha escrito:

Since set seems to be broken, I find myself wanting to implement this metafunction. It need work only for vectors:

// erase all occurrences of T in Seq template struct eraseVal;

After several hours and tens or hundreds of thousands of lines of error messages from three compilers, I am unable to get it working.

My attempt to do it the "right" way looks like this, and I must warn you in advance that it's not pretty, unless you like an approach that calls find three times with the same arguments:

template struct eraseVal : mpl::eval_if< boost::is_same::type, typename mpl::end<Seq>::type>, typename mpl::identity<Seq>::type, eraseVal::type, typename mpl::next::type>::type >::type, T> > {};

My more procedural attempt looks better (to me), but it still doesn't compile:

template struct eraseVal { typedef typename mpl::find::type iter; typedef typename mpl::eval_if< boost::is_same, typename mpl::identity<Seq>::type, typename eraseVal::type, T>::type > type; };

God I miss iteration. And I really wish emacs would match angle brackets in C++ mode, sigh.

Can somebody please help me out?

Hello Scott, I see several problems with your procedural approach: the first one is the ::type added to mpl::identity<Seq> and eraseVal<...> in the expression typedef typename mpl::eval_if< boost::is_same<...>, typename mpl::identity<Seq>::type, typename eraseVal<...>::type > type; Since eval_if is designed precisely to defer invocation of its argument metafunctions the correct mode of use is typedef typename mpl::eval_if< boost::is_same<...>, typename mpl::identity<Seq>, typename eraseVal<...> > type; so that infinite recursion on eraseVal is avoided. But even after dropping those ::type's you still get errors like this: [...]/boost/mpl/next_prior.hpp: In instantiation of `boost::mpl::next': [...] [...]/boost/mpl/next_prior.hpp:31: no type named `next' in `struct mpl_::void_' The problem now is that, although the eraseVal<...> expression is only invoked (i.e. its nested ::type calculated) when strictly necessary --this is what eval_if is used for--, its first argument is computed *always*, regardless of what branch of the eval_if<...> expr is finally selected: typename eraseVal< typename mpl::erase, T> See the mpl::erase<...> thing? When no more T's are left in the sequence, iter points to the end of Seq, and mpl::next<iter>::type is an *invalid* expression (Incidentally, a ::type is missing for mpl::erase<...>, but this is not the root of the problem here.) So, what we need if to add a layer of indirection so as to not invoke the arguments to the recursive call to eraseVal except when strictly necessary, like for instance as follows: template struct eraseVal { template<typename Iter> struct eraseValRecurse: eraseVal::type,T> {}; typedef typename mpl::find::type iter; typedef typename mpl::eval_if< boost::is_same, mpl::identity<Seq>, eraseValRecurse<iter> >::type type; }; This works, but it is quite inefficient, as mpl::find, which computes in linear time, is called for every element of Seq. The same thing can be done in one pass through the sequence as follows: template struct eraseVal2:mpl::copy_ifmpl::_,T > > {}; Please find attached a compilable snippet showing both algorithms at work. HTH, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo